Various medical diagnostic apparatus require that aspects of a patient's respiration, such as inspiration volume, be measured. Illustrative of such apparatus are heart rate variability assessment monitors which can be used in the diagnosis and treatment of various disease states.
Conventional inspiration volume measurement techniques have required the use of face masks or mouthpieces which are invasive and often uncomfortable. One noninvasive technique for measuring a patient's inspiration volume utilizes an extensible conductor adapted for wrapping around the patient's chest in the form of a loop. The conductor is connected to an electrical circuit with which its inductance is measured. One such circuit is described in U.S. Pat. Nos. 4,815,473 and 4,308,872 of Respitrace Corporation, in which the conductive loop provides the inductor in an LC oscillator circuit. The LC oscillator circuit generates a signal having a frequency which varies in accordance with the inductance of the loop. Since the loop inductance varies with variations in the cross-sectional area encompassed by the loop which, in turn, varies with the patient's inspiration volume, the frequency of the oscillator signal varies with the patient's inspiration volume. The frequency signal is converted into a voltage signal having an amplitude proportional to the loop inductance and thus, to the patient's inspiration volume.
While the circuit described in the '473 and '872 patents overcomes drawbacks associated with conventional invasive inspiration volume measurement techniques, the measurement accuracy of the circuit may be limited. This is because the output signal of the circuit is not consistently linearly related to the patient's inspiration volume. A further drawback of the circuit disclosed in these patents is the relatively large number of components required, which lessens its desirability due to manufacturing and cost considerations.
In certain applications, it is advantageous to have the patient breathe in a particular manner when measuring inspiration volume. As one example, in neuropathy diagnosis, it is desirable that the patient breathe at a predetermined, constant frequency, such as on the order of six breaths/minute, or 0.10 Hz. However, even with specific instructions, it is difficult to ensure that the patient will breathe at the constant, predetermined frequency (i.e., to ensure "patient compliance" with a desired breathing pattern). Further, even if the patient complies closely with the desired breathing pattern, a patient's breathing often varies between tests, rendering the results of long-term testing less meaningful due to inter-test variability.
In an effort to enhance patient compliance and thus to reduce inter-test variability, some conventional systems include interactive mechanisms by which the patient is instructed to breathe in accordance with a particular pattern. For example, in a cardiac risk assessment monitor sold under the product name HRView.TM. versions I and II by Boston Medical Technologies, Inc., different audio tones are used to indicate to the patient when to inhale and when to exhale. While this arrangement improves patient compliance and reduces inter-test variability, an even more effective way of ensuring patient compliance with a particular breathing pattern is desirable.